Tire Size Calculator: Diameter, Circumference and Speedometer Error
Enter the three numbers from the tire sidewall (width, aspect ratio, rim diameter) and a second size to compare. You get the dimensions and the speedometer error.
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Formula
- Sidewall height:
width x aspect / 100(mm) - Overall diameter:
2 x sidewall + rim x 25.4(mm; divide by 25.4 for inches) - Circumference:
pi x diameter - Revolutions per mile:
1,609,344 mm / circumference in mm - Speedometer error:
(new diameter / old diameter - 1) x 100
Worked example
225/45R17: sidewall 225 x 45 / 100 = 101.25 mm. Diameter = 2 x 101.25 + 17 x 25.4 = 634.3 mm = 24.97 in. Circumference = pi x 634.3 = 1992.7 mm = 78.45 in, which is 1,609,344 / 1992.7 = 807.6 revolutions per mile.
Compared with 245/40R18 (653.2 mm): 653.2 / 634.3 = 1.0298, so the speedometer error is +2.98 percent and a reading of 60 is a true 61.8.
Frequently asked questions
How do I read a tire size like 225/45R17?
225 is the tread width in millimetres, 45 is the sidewall height as a percentage of that width, and 17 is the wheel diameter in inches. The sidewall is 225 x 0.45 = 101.25 mm.
How do I calculate the overall diameter of a tire?
Add twice the sidewall height to the rim diameter in millimetres (inches x 25.4). For 225/45R17: 2 x 101.25 + 17 x 25.4 = 634.3 mm, which is 24.97 inches.
How much speedometer error does a different tire size cause?
The error equals the new diameter divided by the old one, minus 1. Going from 225/45R17 (634.3 mm) to 245/40R18 (653.2 mm) gives +2.98 percent, so when the speedometer reads 60 you are really doing about 61.8.
Is a few percent of difference safe?
Many manufacturers suggest staying within about 3 percent of the original diameter, but check your vehicle manual. Different sizes also change gearing, clearance and load rating, so this is an estimate for comparison only.